TEM sample holder

ABSTRACT

A TEM sample holder is formed by cutting the TEM sample holder form from a coupon in a press. The cutting at the same time joins the tip point of a nano-manipulator probe tip with the formed TEM sample holder. The tip point of the probe has a sample attached for inspection in a TEM. The cutting process also creates a gap in the sample holder to allow for FIB milling of the specimen.

CLAIM FOR PRIORITY AND INCORPORATION BY REFERENCE

This patent application claims the priority of U.S. application Ser. No.10/896,596, filed Jul. 22, 2004, further claiming priority from U.S.provisional patent application No. 60/519,046, filed Nov. 11, 2003.Application Ser. No. 10/896,596 is incorporated into this divisionalapplication by reference.

TECHNICAL FIELD

This disclosure relates to the use of focused ion-beam (FIB) microscopesfor the preparation of specimens for later analysis in the transmissionelectron microscope (TEM), and apparatus and methods to facilitate theseactivities.

BACKGROUND

The use of focused ion-beam microscopes has become common for thepreparation of specimens for later analysis in the transmission electronmicroscope (TEM). The structural artifacts, and even some structurallayers, in the device region and interconnect stack of currentintegrated-circuit devices can be too small to be reliably detected withthe secondary electron imaging in a Scanning Electron Microscope (SEM),or FIB, which offers a bulk surface imaging resolution of approximately3 nm. In comparison, TEM inspection offers much finer image resolution(<0.1 nm), but requires electron-transparent (<100 nm thick) sections ofthe sample mounted on 3 mm diameter grid disks.

Techniques were later developed for cutting out and removing specimensfor examination that required little or no preliminary mechanicalpreparation of the initial semiconductor die sample before preparationin the FIB. These lift-out techniques include an “ex-situ” method thatis performed outside the FIB chamber, and “in-situ” methods performedinside the FIB.

The process of in-situ lift-out can be simplified into three successivesteps. The first is the excision of the sample using focused ion-beammilling and extraction of the sample from its trench. The second is the“holder-attach” step, during which the sample is translated on theprobe-tip point to the TEM sample holder. Then it is attached to the TEMsample holder (typically with ion beam-induced metal deposition) andlater detached from the probe-tip point. The third and final step is thethinning of the sample into an electron-transparent thin section usingfocused ion beam milling.

A significant portion of the total time involved in completing a TEMsample with in-situ lift-out is spent during the holder-attach step. Therelative amount of time involved depends on the amount of time requiredto mechanically isolate the lift-out sample from the initial bulk sample(ion beam milling rate), but will vary between 30% to 60% of the totaltime for TEM sample preparation. In order to eliminate the holder-attachstep, it would be desirable to directly join the probe-tip point withthe sample attached to the material that will form the TEM sampleholder.

DRAWINGS

FIG. 1 is a plan view of the TEM coupon of the preferred embodiment,where the sample holder has a ring shape and one probe tip is intendedfor use.

FIG. 2 is a plan view of the TEM coupon of FIG. 1, showing a probe tippositioned across it before embedding and cutting.

FIG. 3 is a plan view of the TEM coupon of an alternate embodiment,where the sample holder has a ring shape and four probe tips areintended for use.

FIG. 4 is a plan view of the TEM sample holder and probe-tipcombination, formed from the TEM coupon of FIG. 1, with a ring openingallowing FIB ion milling of the top surface of the lift-out sample.

FIG. 5 is a plan view of the TEM sample holder and the four probe-tipcombination, formed from the TEM coupon of FIG. 2, with a ring openingallowing FIB ion milling of the top surface of the lift-out sample.

FIG. 6 is a cross-sectional view of a probe-tip point embedded in a TEMcoupon.

FIG. 7 is a cross-sectional view of a probe-tip point attached to a TEMcorrugated coupon via electrical or thermal bonding.

FIG. 8 is a cross-sectional view of a probe-tip point attached to a TEMsample holder using adhesive.

FIG. 9 is a cross-sectional view of a probe-tip point bonded to the TEMsample holder material with a CVD or evaporated material.

FIG. 10 is a partial cross-sectional view of the press and the shearpunch of the preferred embodiment.

FIG. 11 is an enlarged partial cross-sectional view of the press and theshear punch of FIG. 10.

FIG. 12 is a perspective view of the shear punch of FIG. 10 and theinner and outer dies.

FIG. 13 is a perspective view of the shear punch of FIG. 10, shownengaging a coupon and a probe tip.

FIG. 14 is a transverse view of the probe-tip point, tip former rod ofthe preferred embodiment positioned above a probe-tip point, and a TEMsample holder, with the probe-tip point embedded in a TEM sample holder.

FIG. 15 is a plan view of the TEM sample holder and probe-tip pointcombination shown in FIG. 4, with the circumferential gap made in thesample holder ring for the backside milling.

FIG. 16 is a plan view of the TEM sample holder and four probe-tippoints combination shown in FIG. 5, with the ring opening made for thebackside milling.

FIG. 17 is a transverse view of a corrugated TEM coupon.

FIG. 18 is a transverse view of a probe-tip point, embedded in acorrugated TEM coupon.

FIG. 19 is a plan view of the TEM coupon of the preferred embodiment,where the sample holder has a rectangular shape and four probe tips areintended for use, with an opening allowing the sample top surface FIBion milling.

FIG. 20 is a plan view of the TEM coupon of the preferred embodiment,where the sample holder has a rectangular shape and four probe tips areintended for use, with an opening allowing backside FIB ion milling ofthe bottom surface of lift-out samples.

DESCRIPTION

The preferred embodiment includes a novel method and apparatus foradjoining a probe tip with attached sample to a TEM sample holder thatreplaces the holder-attach step of the conventional method. In thepreferred embodiment, this mechanical process is performed outside thevacuum chamber, although it could be performed inside the FIB chamber aswell. In the preferred embodiment, the first step of the in-situlift-out procedure (the excision of the sample) is completed in the FIB,and the probe-tip point with the sample attached is then removed fromthe FIB chamber. This removal can be accomplished by a number of means,including but not limited to, removal of the probe tip and attachedsample through the sample door of a FIB equipped with a door,translation of the probe tip and attached sample through a vacuumairlock on the nano-manipulator device, or the translation of theprobe-tip point and attached sample in a cassette that passes through avacuum airlock on the FIB chamber. All but the first means listed do notrequire that the FIB vacuum chamber be vented to atmosphere, whichoffers cycle time reduction and long-term equipment reliabilityadvantages.

The Coupon

In the preferred embodiment, the probe-tip point (160) of anano-manipulator probe (150) is attached to a TEM coupon (100) by acombined mechanical forming and cutting operation. As shown in FIG. 1,the TEM coupon (100) is a sheet of material of approximately the samethickness as the final sample holder (170). The TEM coupon (100)contains the shape of the final sample holder (170) (the “TEMpre-form”), although this pre-form has not yet been completelymechanically isolated. Most of the final shape of the typical 3 mm TEMsample holder (170) can be created in the sheet in advance, asconsumable coupons (100). The pre-form is still attached to the coupon(100) with tabs, lands, or other sections of the sample holder material(120). The pre-form has a ring (180) that will be a part of the finalTEM sample holder (170). The ring (180) is thus defined by a C-shapedhole (135) in the coupon (100). The mouth of the C-shaped hole (135) isthe attaching land (120). Other enclosing shapes, such as rectangles,may also be used.

The holder material is preferably soft copper, but may also bemolybdenum, aluminum, gold, silver, nickel or beryllium, if appropriateto the application. The coupon (100) orients the sample holder form(170) and holds it in place during the mechanical steps of the isolationprocess, described below. FIG. 2 shows a nano-manipulator probe tip(150) placed across the coupon (100). The probe (150) has a probe-tippoint (160) that holds a sample (140) for analysis. Typically, theprobe-tip point (160) is a fine tungsten needle.

The TEM coupon (100) may also be fabricated from a material harder thancopper, such as molybdenum or it may have a surface structure thatfacilitates the mechanical embedding of the probe-tip point (160) in thecoupon material. A good example is a surface structure with corrugations(175) that have a period approximately the same or less than theprobe-tip point (160) diameter. FIGS. 7 and 17 show cross-sections of acorrugated structure. In FIG. 7, the corrugation period is about halfthe diameter of the probe-tip point (160). The corrugations (175) may beperiodic, such as continuous rows or ridges roughly aligned in thedirection of the probe-tip point, rows of individual posts, ornon-periodic free-form elevations. These structures can be easilydeformed to lock the probe-tip point (160) in place.

The remaining tabs, or lands (120) of the coupon material, which connectthe partially formed TEM sample holder (170) to the coupon (100), aresevered during the combined mechanical forming and cutting operation,described below. The TEM sample holder (170) is preferably produced inthe shape of a ring (180) with a circumferential gap (190) to enablelater FIB ion milling of either top or bottom surface of the sample(140) in the plane of the TEM sample holder (170), thus producing anelectron-transparent thin section that would be approximately parallelto the plane of the TEM sample holder (170). Other shapes that allow fora circumferential gap (190) in the ring (180) of the formed TEM sampleholder (170) may also be used. FIGS. 19 and 20, for example, shows a TEMsample holder (170) having two gaps (190), where the shape of the TEMsample holder (170) is rectangular.

FIGS. 4, 5, and 19 show TEM sample holders (170) with probe-tip points(160) mounted for top-side ion milling of samples (140). FIGS. 16 and 20show TEM sample holders (170) with probe-tip points (160) mounted forback-side milling of samples (140).

Methods of Forming the TEM Sample Holder

The probe-tip point (160) with the sample (140) attached can be joinedto the material that will form the TEM sample holder (170), so as topreserve the attachment between the sample (140) and the probe-tip point(160), and prevent the probe-tip point (160) and sample (140) fromseparating from the TEM sample holder (170) during transportation,storage or inspection in the TEM. The assembly should not interfere withthe normal operation of the TEM, or other intended analyticalinstrument, and should survive well in the internal environment of theTEM, or other intended analytical instrument.

FIGS. 6–9 and 18 show methods for joining the probe-tip point (160) tothe TEM coupon (100). FIG. 6 is a cut-away view of mechanicaldeformation of the material of the coupon (100) or probe-tip point(160), or both. FIG. 7 depicts electrical or thermal bonding (320), suchas welding, of the probe-tip point (160) to the coupon (100). FIG. 7also shows corrugations (175) in the TEM sample holder material; in thiscase the corrugation period is about the same as the diameter of theprobe-tip point (160). FIG. 8 shows bonding the probe-tip point (160) tothe TEM sample holder (170) material with a suitable glue or adhesive(330). FIG. 9 shows bonding the probe-tip point (160) to the TEM sampleholder (170) material with a CVD or evaporated material (340).

Once the TEM sample holder (170) with one or more probe-tip points (160)with samples (140) attached to it has been created, it can be returnedto the FIB for the final thinning operation, during which the desiredportion of the lift-out sample (140) or samples is thinned to electrontransparency (typically 50–250 nm). This final thinning can be performedin an off-line FIB to maximize throughput of the in-line FIB and to takeadvantage of the efficiency, expertise and dedicated resources of theoff-line FIB lab that can be located outside the clean room. However, ifthe apparatus for attaching a sample to a TEM sample holder is locatedinside the FIB, the final thinning operation can be performedimmediately.

In an alternative method, the final thinning step can be performed inthe FIB after the lift-out step and before the probe-tip point (160)with the sample (140) attached is removed from the FIB for attachment tothe TEM sample holder outside the FIB. In this method, it is notrequired to return the mechanically formed TEM sample holder (170) withthe sample (140) attached, to the FIB for final thinning. However, thefinal thinning process requires the additional time in the initial FIB.In this method, the probe-tip point (160) with the sample (140) attachedis translated to a suitable location in the FIB, and the ion beam in theFIB is then used to perform the final thinning step. Then, the probe-tippoint (160) with the thinned sample (140) attached is removed from theFIB and attached to the TEM sample holder (170) using the mechanicalforming and cutting process described above. It is recommended, but notrequired, to stabilize the probe-tip point (160) mechanically to reduceany vibration in the probe-tip point (160) relative to the FIB chamberto an acceptable level, or to reduce any mechanical drift of theprobe-tip point (160) relative to the FIB chamber. The probe-tip point(160) with the sample (140) attached can be mechanically stabilized bymaking mechanical contact between the probe-tip point (160) and asuitably stable surface in the FIB, or between the sample (140) and asuitable surface or object in the FIB. For example, the edge or a cornerof a mechanical structure attached to the FIB stage, and the probe-tippoint (160) can be brought together into mechanical contact. Or, thebottom edge of the sample (140) can be brought into mechanical contactwith the surface of the sample stage or any stable mechanical objectattached to the sample stage (e.g., the surface of the wafer). Thestable object can be rigid, or can be deformable by plastic or elasticdeformation, to accept the shape of the probe-tip point (160) orlift-out sample (140) and further dampen any relative mechanicalvibration in the probe-tip point (160).

In another alternative method, the final thinning step can be performedin the FIB after the lift-out step and before the probe-tip point (160)with sample (140) attached is joined to the TEM sample holder inside theFIB vacuum chamber. In this method, the probe-tip point (160) with thelift-out sample (140) attached is translated to a suitable location inthe FIB, and the ion beam in the FIB is then used to perform the finalthinning step. Then, the probe-tip point (160) with the thinned sample(140) can be attached to the TEM sample holder (170) inside the FIBvacuum chamber using the mechanical forming and cutting processdescribed above. In this method, the apparatus for attaching a sample toa TEM sample holder is located inside the same FIB vacuum chamber.Hence, the in-situ lift-out, the attachment of a probe-tip point with asample attached to it to a TEM sample holder, and the final thinningoperation can be performed as steps of one process inside the FIB vacuumchamber.

Sample Holder Forming Apparatus

FIGS. 1 and 2 show a TEM coupon (100), as described above. The land(120) that connects the sample holder portion (170) of the coupon (100)to the rest of the coupon (100) will be severed to form the TEM sampleholder (170) during the cutting and forming operation. The thickness ofthe coupon (100) is determined by the thickness required to embed andmechanically lock the probe-tip point (160) in the coupon (100) materialand still provide for sufficient mechanical strength of the final sampleholder (170) to prevent unwanted folding or separation of the TEM sampleholder (170) at the probe-tip point (160) embedding location. Forexample, for the case of a 125 μm (0.005″) diameter tungsten probe-tippoint (160), a thickness of 250–500 μm (0.010″–0.020″) of copper isappropriate for the coupon (100). Both the sample holder (170) materialand the surrounding coupon (100) material are slightly recessed in aprobe-tip point cut-off zone (130) to allow space for the cuttingsurfaces to cut the probe-tip point (160) without leaving any portion ofthe severed probe-tip point (160) extending beyond the 3 mm outsidediameter of a standard TEM sample holder (170) or extending beyond theoutside border of a standard TEM sample holder (170) of any othersuitable shape.

Alignment holes (110) are included to permit alignment of the coupon(100) in the mechanical apparatus that performs the cutting and formingoperation. In the case of a C-shaped TEM sample holder (170), theprobe-tip point clearance slot (125) (FIG. 3) is a straight slot throughthe coupon (100), radiating outward from the center of the TEM sampleholder (170) beyond the outer diameter of the TEM sample holder (170),that provides clearance for the probe-tip point (160) to permitalignment of the probe-tip point (160) along the surface of the TEMsample holder (170) before the cutting and forming operation.

During the cutting and forming operation, a TEM sample holder (170) iscut from the coupon (100) (FIG. 4). As discussed above, the TEM sampleholder (170) can be produced in a C-shape form, or other shape having acircumferential gap (190) to enable later FIB ion milling of thelift-out sample in the plane of the TEM sample holder (170) to producean electron-transparent thin section approximately parallel to the TEMsample holder (170) plane, or in any other shape allowing the sameprocess. For later milling of the top surface of a sample (140), the gap(190) can be cut from the form at the mouth of the C-shaped hole (135),defined by the land (120) connecting the form to the coupon (100). Forlater milling of the bottom surface of a sample (140), the gap can becut from the form at a location approximately opposite the mouth of thehole (135).

During the cutting and forming operation, the harder tungsten probe-tippoint (160) is pressed into the softer material of the TEM sample holder(170), and the portion of the probe-tip point (160), extending outsidethe outer diameter of the 3 mm TEM sample holder shape (170), is cutoff. The TEM sample holder (170) material is induced to plasticallydeform so that the copper material mechanically surrounds the probe-tippoint (160) to lock it in place (FIG. 6).

FIGS. 10–13 show a typical process for the cutting and formingoperation. The operator places the TEM coupon (100) on the outer die(280) (this operation can be performed by hand, if this operation isperformed outside the FIB, or automatically, if it is performed insidethe same FIB vacuum chamber) and aligns every probe-tip point (160) insuch a way, that every probe-tip point (160) is aligned with theprobe-tip point clearance slot (125), and the sample (140), attached toa probe-tip point (160), is oriented parallel to the plane of the TEMsample holder (170). The inner die (290) and the outer die (280) bothsupport the sample holder (170) and the probe-tip point (160). Onceevery probe tip is secured, the operator positions by hand orautomatically the probe tip or probe tips, TEM coupon (100), and allsupporting hardware under the main mounting block (220), and actuates apneumatic switch (310), causing the main mounting block (220) andattached hardware to travel downward under the action of an actuator(300) located above the main mounting block (220). The actuator (300) ispreferably pneumatic, but hydraulic or electrical actuators may also beused. There is also the exhaust line (305) for pneumatic actuators.

FIGS. 11–13 show the forming and cutting operation as the main mountingblock (220) moves downwards. The former rod (250) contacts everyprobe-tip point (160) and presses it down into the TEM sample holder(170) material. This continues until the TEM sample holder and theprobe-tip point interface build up enough resistance to overcome theforce of the hold down spring (230). The hold down spring (230) force isset with a spring adjustment screw (240) to the desired force to ensurethat every probe-tip point (160) is pressed fully into the TEM coupon(100). The former rod (250) includes one or more teeth (260) that flowthe holder material around the probe-tip point (160) encasing it as itis pressed down (FIG. 14).

Once resistance to the spring (230) is overcome and the former rod (250)movement is stopped, the shear punch (270) continues its traveldownward, using the support of both the inner die (290) and the outerdie (280) to shear every probe-tip point (160) at the desired length,sever the tab (120) connecting the TEM sample holder (170) from the restof the TEM coupon (100) and create the C-shaped opening, or the openingof any other suitable shape, in the holder (170). The operator thenreleases a pneumatic switch to return the main mounting block (220) andattached hardware to its original position, leaving the TEM sampleholder (170) separated from the TEM coupon (100) and containing one ormore probe-tip points (160) with the samples (140) attached.

Since those skilled in the art can modify the specific embodimentsdescribed above, I intend that the claims be interpreted to cover suchmodifications and equivalents.

1. A TEM sample holder, the TEM sample holder comprising: a ring, thering having a circumferential gap; the circumferential gap formed bycutting the circumferential gap from a TEM sample holder form.
 2. TheTEM sample holder of claim 1, where: the ring is defined by a hole in acoupon for preparing a TEM sample holder; the hole having a mouth; wherethe gap is formed by cutting the circumferential gap from a TEM sampleholder form at the mouth of the hole, so as to allow FIB milling of thetop surface of a sample.
 3. The TEM sample holder of claim 1, where: thering is defined by a hole in a coupon for preparing a TEM sample holder;the hole having a mouth; where the gap is formed by cutting thecircumferential gap from a TEM sample holder form at a locationapproximately opposite the mouth of the hole, so as to allow FIB millingof the bottom surface of a sample.
 4. The TEM sample holder form ofclaim 1, where the cutting of the TEM sample holder form comprisespressing the TEM sample holder form between two dies.
 5. The TEM sampleholder of claim 1, further comprising: one or more probe-tip pointsembedded in the ring.
 6. The TEM sample holder of claim 5, where theprobe-tip points are embedded in the ring by applying pressure to thering and the probe-tip points, so as to cause plastic flow of the ringabout the probe-tip points.
 7. The TEM sample holder of claim 5, wherethe one or more probe-tip points are attached to the ring by welding. 8.The TEM sample holder of claim 5, where the one or more probe-tip pointsare attached to the ring by chemical-vapor deposition.
 9. The TEM sampleholder of claim 5, where the one or more probe-tip points are attachedto the ring by adhesive.
 10. The TEM sample holder of claim 5, where thesurface of the sample holder has corrugations for locking the one ormore probe-tip points to the ring.
 11. The TEM sample holder of claim10, where the corrugations are periodic, having a period approximatelythe same or less than the diameter of the one or more probe tip points.12. A TEM sample holder, the TEM sample holder comprising: a ring, thering having an approximately rectangular shape; the ring having acircumferential gap; the circumferential gap formed by cutting thecircumferential gap from a TEM sample holder form.
 13. The TEM sampleholder of claim 12, where: the ring is defined by a hole in a coupon forpreparing a TEM sample holder; the hole having a mouth; where the gap isformed by cutting the circumferential gap from a TEM sample holder format the mouth of the hole, so as to allow FIB milling of the top surfaceof a sample.
 14. The TEM sample holder of claim 12, where: the ring isdefined by a hole in a coupon for preparing a TEM sample holder; thehole having a mouth; where the gap is formed by cutting thecircumferential gap from a TEM sample holder form at a locationapproximately opposite the mouth of the hole, so as to allow FIB millingof the bottom surface of a sample.
 15. The TEM sample holder form ofclaim 12, where the cutting of the TEM sample holder form comprisespressing the TEM sample holder form between two dies.
 16. The TEM sampleholder of claim 12, further comprising: one or more probe-tip pointsembedded in the ring.
 17. The TEM sample holder of claim 16, where theone or more probe-tip points are embedded in the ring by applyingpressure to the ring and the probe-tip points, so as to cause plasticflow of the ring about the probe-tip points.
 18. The TEM sample holderof claim 16, where the one or more probe-tip points are attached to thering by welding.
 19. The TEM sample holder of claim 16, where the one ormore probe-tip points are attached to the ring by chemical-vapordeposition.
 20. The TEM sample holder of claim 16, where the one or moreprobe-tip points are attached to the ring by adhesive.
 21. The TEMsample holder of claim 16, where the surface of the sample holder hascorrugations for locking the one or more probe-tip points to the ring.22. The TEM sample holder of claim 21, where the corrugations areperiodic, having a period approximately the same or less than thediameter of the one or more probe tip points.
 23. A TEM sample holder,the TEM sample holder comprising: a sheet of material; the sheet ofmaterial having substantially rectangular dimensions; an opening in thesheet of material; a path through the sheet of material; the pathconnecting the opening to the edge of the sheet, thereby forming a TEMsample holder form allowing for FIB milling of a sample within theopening.
 24. The TEM sample holder of claim 23, further comprising: oneor more probe-tip points embedded in the sheet of material.
 25. The TEMsample holder of claim 24, where the probe-tip points are embedded inthe sheet of material by applying pressure to the sheet of material andthe probe-tip points, so as to cause plastic flow of the sheet ofmaterial about the probe-tip points.
 26. The TEM sample holder of claim24, where the one or more probe-tip points are attached to the sheet ofmaterial by welding.
 27. The TEM sample holder of claim 24, where theone or more probe-tip points are attached to the sheet of material bychemical-vapor deposition.
 28. The TEM sample holder of claim 24, wherethe one or more probe-tip points are attached to the sheet of materialby adhesive.
 29. The TEM sample holder of claim 24, where the surface ofthe sample holder has corrugations for locking the one or more probe-tippoints to the sheet of material.
 30. The TEM sample holder of claim 29,where the corrugations are periodic, having a period approximately thesame or less than the diameter of the one or more probe tip points. 31.The TEM sample holder of claim 23 further comprising: a plurality ofopenings in the sheet; a plurality of paths through the sheet; each pathconnecting an opening to the edge of the sheet.